In recent years, applications employing near-field wireless power and/or data transmission and/or communication systems, such as commercial electronics, medical systems, military systems, high frequency transformers, microelectronics including nanoscale power and/or data transfer or microelectromechanical systems (MEMS) thereof, industrial, scientific and medical (ISM) band receivers, wireless sensing and the like, have been limited in achieving optimal performance because the antennas (also referred to as resonators) utilized in these systems have relatively low quality factors.
The relatively low quality factors of these wireless transmission and/or communication systems are mainly due to higher resistive losses caused by a phenomenon known as the “skin effect.” Generally, skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor such that the current density is more predominant near the surface of the conductor with the remaining conductor body ‘unused’ relative to electrical current flow. The remaining conductor body is ‘unused’ relative to electrical current flow because the current density typically decays with distance therewithin away from the surface of the conductor. The electric current flows mostly near the surface, and is referred to as the “skin” of the conductor. The depth at which current flows from the surface is referred to as the “skin depth.” The “skin depth” then defines the electrical signal conducting path that is active in transmission and/or communication, while the conductor is defined as the body that is capable of conducting an electrical signal.
In systems employing wireless power and/or data transmission and/or communication, the skin effect phenomenon generally causes energy loss as current flows through the antenna wire and circuit. Higher resistive loss at high frequencies is a problem faced by most electronic devices or appliances. Skin effect becomes more prevalent when operating frequency increases. With higher frequencies, current that normally flows through the entire cross section of the wire forming the antenna becomes restricted to its surface. As a result, the effective resistance of the wire is similar to that of a thinner wire rather than of the actual diameter through which the current could be distributed. A wire exhibiting tolerable resistance for efficient performance at low frequency transitions into a wire of unacceptable resistance at high frequency. The transition from tolerable to unacceptable resistance translates into an inefficient power and/or data transmission and/or communication system that is unable to conduct an electrical signal as needed in particular applications. Additionally, today's antenna designs do not resolve these inefficiencies, and, in some cases, exacerbate the inefficiencies of a wireless power and/or data transmission and/or communication system. Although not exhaustive, typical applications limited by current antenna technology include, for example, radio frequency identification (RFID), battery charging and recharging, telemetry, sensing, communication, asset tracking, patient monitoring, data entry and/or retrieval and the like. Overheating of system components, rate and accuracy of data retrieval, rate of energy delivery, transmission distance constraints, and transmission misalignment limitations are other serious problems in wireless power and/or data transmission and/or communication applications.
In applications of Implanted Medical Devices (IMDs), such as pacemakers, defibrillators and neuromodulation or neuromuscular stimulation devices, there is a desire to minimize battery recharge time. Faster battery recharge time reduces, for example, patient duration of discomfort, inconvenience, and potential for injury. If antennas have less resistive losses, battery recharge could be accomplished from greater distances and with higher tolerance to antenna misalignment or disorientation without compromising performance. Precise orientation and alignment is known to be difficult to achieve, especially for obese patients. Additionally, and/or alternatively, if structures of smaller sizes can be designed and practically manufactured while maintaining the performance characteristics required for successful system operation, then the overall dimensions of IMD's could be decreased.
In RFID applications, such as supply chain management, product authenticity, and asset tracking, there is a need to increase read range, increase read rates, improve system reliability and improve system accuracy. At high frequency for example, read range is at most three feet which is generally insufficient for pallet tracking. Ultra high frequency readers enable greater read distances of eight to ten feet, however, they introduce other performance issues like signals that are reflected by metal or are absorbed by water, or display unreadable, null spots in read fields. Increased read range requires concentrated power to facilitate reflecting back the signal for better performance, hence, a more efficient structure could help solve these issues.
In applications requiring efficient low loss coils which need to maintain resonance under harsh conditions, conventional wire-based antennas could be deformed. It is well known that any deformation of the wire cross-section will lead to a change in inductance and possibly resistance, which in turn will change the resonance frequency of the antenna and consequently may increase overall system resistance. Improved methods of manufacturing these types of structures that reduce the potential for compromising deformation could eliminate this problem. The present teachings include methods of manufacture that include both rigid structure designs and fixed flexible structure designs.
Litz wires were developed, in part, in an attempt to address the issues discussed above. However, Litz wires are generally insufficient for use in high frequency applications, and are therefore generally not useful in applications having operating frequencies above about 3 MHz. A Litz wire is a wire consisting of a number of individually insulated magnet wires twisted or braided into a uniform pattern, so that each wire strand tends to take all possible positions in the cross-section of the entire conductor. This multi-strand configuration or Litz construction is designed to minimize the power losses exhibited in solid conductors due to “skin effect”. Litz wire constructions attempt to counteract this effect by increasing the amount of surface area without significantly increasing the size of the conductor. However, even properly constructed Litz wires exhibit some skin effect due to the limitations of stranding. Wires intended for higher frequency ranges generally require more strands of a finer gauge size than Litz wires of equal cross-sectional area but composed of fewer and larger strands. The highest frequency at which providers of Litz wires offer configurations capable of improving efficiencies is about 3 MHz. There is currently no solution for applications with operating frequencies beyond this 3 MHz maximum frequency limit.
Hence a need exists for an improved high efficiency structure design and method of manufacture that reduces the intrinsic resistive losses of the structure, and in particular reduces intrinsic resistive losses of the structure at high frequencies to achieve high quality factors.